This invention relates to a positive electrode active material for use in nonaqueous electrolyte secondary cells, in particular, lithium secondary cells, as well as starting materials for said active material, and a process for producing the same.
Lithium cobalt oxide (LiCoO.sub.2) is currently used as the positive electrode active material in lithium secondary cells. This active material has a capacity of 120-140 mAh/g and a cycle characteristic (life) of about 500 cycles. As recent models of electronic devices have higher performance in smaller sizes of a cellular (cordless) unit, it is required to use smaller and lighter cells as drive power supplies. One of the approaches proposed to meet this requirement is substitution of LiNiO.sub.2 as the active material for positive electrode. LiNiO.sub.2 has a high capacity but, on the other hand, it is short-lived. With a view to solving this problem, an attempt is being made to add elements other than Ni but no satisfactory results have been attained. It has also been proposed to optimize the particle size, granulate the active material and increase its density but with only limited success. Aside from the short life, LiNiO.sub.2 has another problem in that as the production scale increases, the characteristics of the product vary not only from one lot to another but also from one portion to another within the same lot, making it difficult to manufacture powders of consistent characteristics.
Only in a small-scale synthesis of lithium complex oxide, homogeneous products can be obtained in ordinary firing furnaces supplied with an oxygen gas or air. Cells using the positive electrode active material synthesized with an increased throughput in order to achieve higher productivity have had problems such as deterioration in charge-discharge characteristics and increase deviation in characteristics.
Under the circumstances, it has been proposed in Unexamined Published Japanese Patent Application No. Hei 5-62678 to effect forced passage of a gas in order to ensure a homogeneous overall reaction despite the increased throughput of the firing operation. According to the proposal, the lithium complex oxides are synthesized in a firing furnace by firing a bed of mixed powders as it is supplied with a forced flow of air or oxygen or a mixture of oxygen and nitrogen which are heated to a specified temperature. The mixed powders are contained in a reaction vessel placed in a firing furnace equipped with an electric heater and a porous ceramic plate supporting the mixed powders is provided in the bottom of the reaction vessel. Compressed air supplied from an external air pump is preheated by a heat exchanger and supplied into the bottom of the reaction vessel such that it is forced through the bed of mixed powders. However, this increases rather than reduces the variations in characteristics.